1. Field of the Invention
The present invention relates to an optical information recording and reproducing apparatus for recording and/or reproducing information by using a light beam.
2. Related Background Art
An optical disk drive which uses an optical disk as a recording medium has been recently attracting notice as an external storage for a computer. In such an optical disk, record tracks are formed spirally or concentrically, and information is recorded on the tracks as record pits. Since the pitch of the tracks is usually 1-2 .mu.m to provide a high recording density, the optical disk drive may be used as a large capacity storage device.
In order to record information on an optical disk or reproduce the information recorded on the optical disk, an optical head for irradiating the optical disk with a laser beam is used. The optical head comprises a semiconductor laser which emits a laser beam and an objective lens actuator which adjusts the irradiation status of the laser beam with respect to the optical disk (at least one of focusing and tracking), and it controls the drive of the optical head by controlling the objective lens actuator so that the laser beam is projected on to a target track on the optical disk.
In order to control the optical head, a high gain and wide band servo system is required, but it is extremely difficult to attain sufficient stability in such a high gain and wide band servo system. Where the servo system comprises a digital control circuit, it is much more difficult to attain sufficient stability than in an analog control circuit because of wasted time due to the operation time for the control operation or the phase turn-around in a high frequency range due to a zero-order holder.
In order to solve the above problems, it has been proposed to construct a control system by building in a repetition compensator which uses a shift register. The repetition compensator has as many shift register stages as the number of times sampling occurs in one period in one rotation of the disk and stores a servo error signal for each sampling operation. At each sampling, it produces the servo error signal of the previous sampling and adds the output signal and a current servo error signal to cancel a periodic variation due to the eccentricity of the disk. This will be discussed in detail later. Thus, since the repetition compensator has a high tracking characteristic with respect to the target, the gain of the digital stabilization compensator which produces the control signal for the objective lens actuator can be significantly reduced. As a result, the control system is stabilized, a higher gain is attained than a system without the repetition compensator, and the tracking characteristic is improved.
However, in the prior art, since the sampling frequency of the repetition compensator and the digital stabilization compensator are set to the same value, such a system has the following problems. The frequency band required for the repetition compensator is several times as high as a rotation frequency of the disk. Since the normal rotation frequency of the disk is about 50 Hz, the upper limit of the required frequency band is approximately several hundreds Hz. On the other hand, the frequency band required for the digital stabilization compensator is up to several KHz. Thus, if the sampling frequencies of both compensators are to be set equally, they should be set to several tens of KHz which conforms to the sampling frequency of the digital stabilization compensator while taking into account the fact that the sampling frequency of the digital control system is usually ten times as high as the frequency band. As a result, the number of stages of the shift register which stores the servo error signals of the rotation period of the disk increases and the circuit configuration becomes extremely complex.
Assuming that the rotation frequency of the disk is 50 Hz (3,000 rpm), the frequency band required for the repetition compensator may be approximately ten times as high as the fundamental frequency which is the rotation frequency of the disk, and the sampling frequency of the repetition compensator may be approximately ten times as high as the frequency band of the repetition compensator, that is, approximately 5 KHz. Where the sampling frequency of the repetition compensator is 5 KHz, the number of stages of the shift register is 100.
On the other hand, if the sampling frequencies of the repetition compensator and the digital stabilization compensator are set equally, the sampling frequency of the repetition compensator is several tens of KHz. For example, when the sampling frequency of the repetition compensator is 50 KHz, the number of stages of the shift register is 1,000, which requires ten times as large a memory capacity as that required in the previous example. Thus, in the prior art, since the sampling frequencies of the repetition compensator and the digital stabilization compensator are equally set, many shift register stages are used and the circuit configuration of the control system is very complex.